Pathophysiology of cardiopulmonary bypass: A current review

The abnormal physiology of cardiopulmonary bypass includes haemodilution, hypothermia, interstitial fluid accumulation, complement activation and depression of immune mechanisms. These factors have many interactions and relationships. Haemodilution ameliorates the adverse effects of hypothermia and the heart-lung machine on organ blood flow, oxygen delivery and renal function. Complement activation produces vasoconstriction, capillary leakage and whole-body inflammatory reaction. Interstitial fluid accumulation is partly due to dilution of plasma oncotic pressure during hemodilution. Fluid accumulation during cardiopulmonary bypass is related to duration on cardiopulmonary bypass, the underlying cardiac disease, patient's age, female sex, obesity, aorto-iliac-femoral occlusive disease, and low ejection fraction. Hypothermia of varying degrees is used during cardiopulmonary bypass. The principal advantage of hypothermia is the lowering of total body oxygen demand. The period of rewarming following hypothermia is a time of accelerated complement activation. For routine coronary bypass operations, perfusion at lower temperatures may be associated with greater morbidity. All components of immune function are depressed following cardiopulmonary bvypass. Immune globulins are diluted, denatured and consumed during cardiopulmonary bypass. Polymorphonuclear leukocytes decrease in number and in function. The function of lymphocyte subgroups is depressed following cardiopulmonary bypass. The reticuloendothelial system undergoes blockage during bypass. The ability of the reticuloendothelial system to ingest circulating bacteria and other microparticles is diminished following cardiopulmonary bypass. Specific protocols for conducting cardiopulmonary bypass to preserve organ function have recently been developed. These specific protocols are designed to decrease the incidence and severity of renal failure and neurologic dysfunction.